Hollow walls, as commonly known, are interior walls generally comprised of gypsum board, plasterboard and the like (wallboards), of specific thicknesses (generally having thicknesses of ⅜″, ½″ and ⅝″), which are mounted on studs. A hollow is accordingly formed behind the relatively thin outer surface materials and between the spacing studs. Other panels such as hollow doors and flexible sheet rock (with a gypsum composition) have thicknesses of as thin as ⅛″ and their thicknesses, usually are in respective increments of ⅛″ (¼″ ⅜″, ½″, and ⅝″). The gypsum or sheet rock and plasterboard walls are of limited structural integrity because of their very thinness or composition (e.g. gypsum board owes much of its strength to the paper used to enclose the compressed powder gypsum) and they will not adequately support items hung thereon with simply nails or even standard screws (except directly into the stud). Nevertheless, there is a need to hang decorative (e.g., paintings) and utility items (e.g., shelving), often in areas distant from the supporting studs. A class of hollow wall anchors has been developed for use in conjunction with a nail or screw, to enhance holding strength. Common anchors of this type, however often have drawbacks. Thus, for example, “plug” anchors (made of metal, plastic or fiber), which are expanded by an inserted screw against the interior gypsum of the wallboard, still rely on a tenuous holding strength provided by compressed powdered gypsum. Other anchors include toggle bolt anchors which cannot be reused and which require pre-drilled holes for installation. Self-installing drive-in molly bolt anchors may damage a wall if not properly installed, particularly if the wall is thin. Many of the hollow wall anchors are susceptible to loosening and failure, particularly with a dynamic load, e.g., removal and replacement of pictures, and typical wall vibration. Furthermore, since the installation is hidden it is often difficult to ascertain if an anchor has been installed properly.
Two other types of hollow wall anchors are generally available for use with hollow walls for supporting loads. The first is the premier high holding strength expansion anchor, exemplified by the Toggler® anchor (available from Mechanical Plastics Corp., New York), which, when positioned, expands behind the inner surface of a wall, to spread the holding load against a greater surface of the wall. This anchor is available in sizes suitable for very thin walls on the order of ⅛ to ¼″ and for wall thicknesses up to about 1″ (dual ½″ boards). However, this type of anchor requires a preformed or predrilled aperture for deployment.
A second type of anchor (available in both plastic and metal) is the auger type anchor (also known as self drilling or self installing screw anchor for use in relatively soft gypsum boards) comprised of a short metal or plastic rod having a single or multiple spike end and a base section with an oversize screw thread. With this anchor, the spike end starts the insertion positioning and boring until the screw threads engage the wall and the anchor is then self-threaded (by engagement with a screwdriver or more often with a screw gun) into the hollow wall gypsum until a front flange engages the wall. The threads provide all the holding strength of the anchor against the interior walls of the aperture being formed.
While placement of the auger anchors is rapid, without the need for a pre-drilled aperture (making them popular with some contractors), these auger anchors, rely only on the screw threads (albeit oversized) to retain the anchor in the wallboard. They are accordingly subject to problems, which can cause premature release under loads near their maximum ratings. This premature release can be caused by excessive torque upon installation (a common occurrence with a screw gun used without care during the installation process), allowing the threads to strip the softer internal plaster laminate or powdered gypsum, thereby weakening the holding strength. This interior disruption is however not discernable to the installer, making failure an insidious unknown. Even with proper installation, subsequent failure of the installation under load can also occur spontaneously over a longer period of time, as a result of vibrations normally occurring in building structures. Often, removal of an inserted screw can cause the anchor to unthread from the wallboard, along with screw, due to the frictional force of the screw thread being greater than that between the anchor and wallboard. This makes reloading the anchor unfeasible since the wall, at such point, has been effectively destroyed.
Recently, embodiments of such self installing anchors such as exemplified, for example in U.S. Pat. No. 5,692,864, have been provided with expandable segments of the forward non-threaded section, which engage the distal side of the wall with screw insertion, but which embodiments are not fully satisfactory with respect to enhanced holding strength and/or positive deployment.
The presently available self installing anchors are also not suitable for hollow walls or threadable panels with thicknesses under ⅜″ since there is very little thread purchase possible. In addition, even the slightly larger (thicker) thin walls only provide a very tenuous support for the thread of such anchors and it is such engagement which provides the entire load support. Holding power of such anchors in such walls, with resistance to tensile pull, is minimal at best.